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Discussion Starter #1
Ok, where am I going wrong? I am trying to calculate the END for trimix 28/25 with a depth of 42m.
I’ve got three different equations, each giving a different result;

END = (D + 10) x (1 – FHe) – 10

END = [(1 – FHe) x ata x 10] – 1

END = [FN2 x (D + 10) / 0.79] – 10


Working through them;

(D + 10) x (1 – FHe) – 10
(42 + 10) x (1 – 0.25) – 10
52 x 0.75 -10
= 41.25

and

[(1 – FHe) x ata x 10] – 1
[0.75 x 5.2 x 10] – 1
39-1
= 38

and

[FN2 x (D + 10) / 0.79] – 10
[0.47 x (42 + 10) / 0.79] – 10
[0.47 x 52 / 0.79] – 10
30.94 – 10
= 20.94

So, what have I screwed up? Is it my maths (highly possible) or have I got the equations wrong???
 

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Bashing head against brick wall
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There are a number of differing calculations dependant upon whether you're treating nitrogen, nitrogen and oxygen, nitrogen and helium, or nitrogen, oxygen and helium as narcotic or just treating it as a mathematical exercise.

These 5 methods will all give you differing END so stick with your agency of choices recommendation for the calculation and you'll be fine.
 

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R.I.P.
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too many ciders have gone past my lips this evening to even attempt looking at the calcs, but i'm sure you'll find something useful here to help:

tools
 

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Always did like HP sauce.....
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OK I follow the 20.94 figure but why are we talking about EAD's too?
 

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Discussion Starter #9
Thanks guys, I think I'll stick with the last formula (Iantd) strange that the second formula (GUE) didn't match up with decoplanner??
 

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Finding The END (Equivalent Narcotic Depth - Oxygen Is Narcotic - ala TDI / IANTD) Of A Mix

[FN2 x (Meters+10) / 0.79] Minus10 = END


Finding The END (Equivalent Narcotic Depth Of A Mix - Oxygen Is Not Narcotic - ala GUE) Of A Mix

[(1-FHe x ATAs) minus 1] x10 = END

Hope that helps ;)


 

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Nigel Hewitt
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Ok, where am I going wrong? I am trying to calculate the END for trimix 28/25 with a depth of 42m
42 meters so 5.2bar deep.

O2 isn't really narcotic so we're only really interested in nitrogen.
28/25 is really 28/25/47
so we have 5.2*0.47 = 2.444bar of nitrogen.

END is let's pretend it's air diving so the nitrogen would be depth_in_bar*0.79
so 2.444/0.79 is 3.1 bar
3.1 bar deep is 21 meters

Trying to learn formula is madness. It always ends in tears.
 
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42 meters so 5.2bar deep.

O2 isn't really narcotic so we're only really interested in nitrogen.
28/25 is really 28/25/47
so we have 5.2*0.47 = 2.444bar of nitrogen.

END is let's pretend it's air diving so the nitrogen would be depth_in_bar*0.79
so 2.444/0.79 is 3.1 bar
3.1 bar deep is 21 meters

Trying to learn formula is madness. It always ends in tears.
I'd agree with that, except the bit about O2 not being narcotic.

it is, but the question is, in reality, does it matter that it is, and should we, therefore, include it in calculations.
 

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Nigel Hewitt
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Nigel that's a brilliant explanation, provided we accept O2 isn't narcotic, and one that saves a great deal of calculation. I shall be using that in future. It's as good as Pigs eat Figs and Peas for Daltons Law.
But 'oxygen is narcotic' is even easier...
Well provided you want it to be the same amount of narcotic as nitrogen which is another debate.

5.2bar deep as before
Gas is 28/25/47 so 28+47 of nasties = 75
so 5.2*0.75 of nasties = 3.9 bar
air is all nasties so that's it (well, divide by 1)
3.9 bar deep is 29 meters.

Oxygen ought to be narcotic based on the lipid solubility stuff that works quite well for anaesthetic gases but O2 is consumed at the point of action and I for one think that is crucial. However until we have a CE marked narko-meter that we can plug into somebody's head and read out that they are 17.5% narked it's all rather subjective.

My point with the examples is that by doing the sums in 'depth in bar' everything becomes simple steps of multiply or divide and is far more obvious plus you can look at the results of each step and say 'yes that should be more/less than the previous number' check. Trying to remember formula with add 1, or was it 10, or was it subtract just adds too many failure points to the maths. It is really simple and doing it by understanding not by memorisation you end up with a feel for the numbers. With this one 75% is nasties (I keep wanting to type 'inerts' but O2 is never inert) depth is 5 and a bit bar. 75% of 5 and a bit is going to be about 4 so about 30 meters. 30 meters is near enough for an END calculation.
 
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I'm glad someone could explain it, thanks Nigel

I got the answer right just didn't explain how I got to it!
 

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Using the Meyer Overton Rule, Oxygen should be around twice as narcotic as Nitrogen because it's around twice a soluble in fat. As Nigel points out, you metabolise it constantly so it's hard to believe that this still applies.

As a rule of thumb - its seems sensible and easy to reckon that it's about as narcotic as N2. Some properly carried out research was done a while back that got a bunch of divers to carry out higher brain fuction type puzzles, such as maths calculations, at depth, while on Nitrox and / or air and there was no difference in impairment recorded between the two, which would tend to support this.

If you accept this - as Nigel also points out - it makes the maths very easy - everything that's not Helium is equally narcotic - just multiply the ATAs by the 1-FHe and that's your narcotic depth :)
 

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The Artist formerly known as 'Robert110csw'
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Using the Meyer Overton Rule, Oxygen should be around twice as narcotic as Nitrogen because it's around twice a soluble in fat. As Nigel points out, you metabolise it constantly so it's hard to believe that this still applies.

As a rule of thumb - its seems sensible and easy to reckon that it's about as narcotic as N2. Some properly carried out research was done a while back that got a bunch of divers to carry out higher brain fuction type puzzles, such as maths calculations, at depth, while on Nitrox and / or air and there was no difference in impairment recorded between the two, which would tend to support this.

If you accept this - as Nigel also points out - it makes the maths very easy - everything that's not Helium is equally narcotic - just multiply the ATAs by the 1-FHe and that's your narcotic depth :)
------------------------------------------

Have you got the details of this study I would love to read it please. I am always suspicious of these studies and claims do you know if it was a bouble blind study ?

Thanks in advance

Regards

RAB
 
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